(almost) Removed DicomSeriesReader!!! *FINALLY*
Moved the DicomSeriesReader as a private legacy asset to
DICOMReader.
Reason: See description of T24540. So we need it for one last use case,
but want to keep the code as private as possible.
Signed-off-by: Ralf Floca <r.floca@dkfz-heidelberg.de>